1. Field of the Invention
The present invention relates to a sputtering apparatus, and more particularly, to an apparatus for forming a magnetic layer.
2. Description of the Related Art
A magnetic hard disk drive (HDD) includes a magnetic disk medium having high coercivity polycrystalline magnetic films thereon and at least a transducer (magnetic head). The transducer includes a magneto-resistive read sensor which reads data from the disk medium and a high magnetic moment writer which writes data on the disk medium.
The writer is a patterned thick magnetic film. Alloys of Fe—Co are typically employed due to their large magnetic saturation (magnetic moment) as the magnetic layer in the writer.
Transducers employed in HDDs are fabricated on 5 to 8 inch diameter wafers made of Al2O3—TiC. A single wafer is cut to give rise to several tens of thousands of transducers. Therefore, uniformity and “low skew” over an entire wafer are very important as they determine the yield. Here, low skew means that the deviation of the direction of magnetic easy axes on a wafer from a predetermined direction is small, i.e., the easy axes are well aligned.
A read sensor is typically made of multiple layers of very thin films, several of which are made of soft magnetic material such as Fe—Co—B and Ni—Fe alloys separated by a nonmagnetic spacer such as Cu or MgO. In read sensors similar to writer films, it is preferred that each easy axis is aligned along a predetermined direction. For both writers and readers, the easy axis alignment is preferably made during deposition, as the effect of relatively low temperature (<300° C.) post annealing is modest, especially in the case of Fe—Co films.
Alignment of the easy axis is usually made by applying an external field to the film while the film is being formed. The external field promotes pair-ordering anisotropy. Therefore, in a sputtering process, a separate magnetic field source is employed so as to expose the substrate surface to parallel fields. The fields may have a small component normal to the substrate surface but are preferably predominantly parallel to the surface plane. For example, when Fe-rich Fe—Co is deposited at near normal deposition angles with no substrate field, an in-plane isotropic film is obtained. The coercivity in any direction on the film plane is essentially constant.
By applying an external field during deposition, uniaxial anisotropy is induced. For example, the coercivity may be ≧25 Oe in the substrate field direction (easy axis) and <1 Oe in the perpendicular direction (hard axis). However, it is difficult to apply parallel substrate magnetic fields in a sputtering apparatus having cathodes with circular or rotating magnetrons, because such magnetrons emit fields that may interfere with the substrate magnetic fields. Moreover, the substrate fields adversely affect film uniformity. The distance between the target and the substrate may be increased to improve film uniformity, but the improvement comes at the expense of significant reduction in deposition rates.
Oblique deposition is also known to promote uniaxial anisotropy in Fe—Co films. For technologically important Permalloy (Ni81Fe19) and high saturation magnetization Ms (e.g. Fe70Co30) films, the easy axis direction changes by 90 degrees at an incidence angle of 45 degrees. For modest incidence angles<45 degrees, the induced easy axis is in the plane of the incidence. Here, incidence angle is defined as an angle between a substrate normal axis and an incidence direction of sputtered particles. For angles greater than 45 degrees, the easy axis is transverse or perpendicular to the plane of incidence. Oblique deposition gives rise to films with anisotropy strength significantly higher than those deposited at near normal angles in the presence of an aligning magnetic field. This is true for a large class of magnetic materials such as Co, Fe, Ni—Fe, Fe—Co, and Fe—Co—B. Very high anisotropy values can be obtained especially at an incidence angle of 70 to 80 degrees. The induced anisotropy is attributed to anisotropic stress or shape anisotropy due to shadowing during deposition. However, obliquely deposited films exhibit reduced density and saturation magnetization density. Modest oblique angle deposition can only be applied when high saturated magnetization density is required.
Forming an undercoat layer with relatively thick seed layers (up to 5 nm) deposited by oblique deposition can result in large Hk for magnetic layers subsequently deposited on the seed layers. Accordingly, both high Hk and density can be achieved for the magnetic layer. Moreover, at modest magnetic layer deposition incidence angles that maintain the saturated magnetization density (<40 degrees for Fe70Co30), even without the benefit of an obliquely deposited seed layer, higher anisotropy strength is obtained compared to near normal incidence deposition in the presence of an aligning field. Oblique deposition is therefore a good candidate for fabricating write-pole films with large Hk.
For applications that do not need the maximum 2.4 T magnetization, such as sensors, higher angles may be employed to access higher Hk values. The Hk dependence with incidence angle is steep at high angles. Therefore, both the incidence angle and its distribution must be controlled to obtain large area films with uniform magnetic properties.
Oblique deposition is also effective for applications that require anisotropic stress. For example, techniques that utilize non-magnetic compression layers to improve self-pinning of one of the magnetic layers of a sensor rely on a cut to transform the isotropic stress into an anisotropic compressive stress perpendicular to the direction of the cut. The stress anisotropy can further be increased by oblique deposition of the compressive layers.
Fe—Co alloys exhibit larger easy axis coercivities than Ni—Fe films. Furthermore, for technologically important high saturation magnetization, Fe-rich Fe—Co alloys, the magnetic anisotropy (and easy axis coercivity) may typically increase with thickness. This is mainly due to grain size increasing with thickness. For sensors, the magnetic films employed are very thin (<5 nm) whereas for write-poles, the films may be thicker than 200 nm. A method to mitigate grain size growth, such as by seed layer deposition and adatom mobility reduction during film formation is therefore advantageous particularly for write-pole film deposition. It is known that Cu, Ni—Fe, or Ru seed layers are effective for grain size reduction in Fe—Co films. To suppress grain coarsening by increasing layer thickness, substrate cooling may be made available.
JP 2005-526179 A discloses a method and an apparatus for depositing magnetic films in the presence of a magnetic field from a physically fixed substrate field source. With a large rectangular target and a smaller round substrate, portions of a cathode magnetron are located far from the substrate such that the stray magnetic fields from the cathode magnet may not to interfere with the substrate magnetic fields. The part of the cathode magnetron that is closer to the substrate has stray fields that are substantially parallel to the field generated by the substrate field source. The substrate or wafer to be processed is translated under the target parallel to the substrate fields and perpendicular to the target length. Intrinsic to the linear translation method, every segment parallel to the target length of a passing substrate surface receives the same flux of incoming sputtered particles. Excellent film thickness uniformity along the translation direction can be expected. And for sufficiently long targets, this arrangement also results in films with good transverse film uniformity and low skew values. Very high deposition rates are accessible and lower rates can be achieved by increasing the substrate translation speed.
JP 2005-526179 A discloses a method for translating a substrate perpendicular to the long side of the elongate target. The uniformity along translating direction can be ensured by translating a substrate along the flux of sputtered particles from the target. A sufficiently long target helps to achieve the improved uniformity along traverse direction. In long targets, in general, elongated erosion pattern can be formed. In long and parallel shaped erosion, sputtering rate is relatively uniform. For a substrate symmetrically arranged under a target, the deposition rate at the center of the substrate is highest. Typically, as for a substrate of 200 mm in diameter and the distance between the substrate and the target of 100 to 200 mm, in order to achieve the thick uniformity of 1 σ<5%, the target size should be twice of the substrate size.
Japan Patent No. 4352104 discloses an oblique sputtering apparatus which uses a rotating target, a substrate stage, and a drum shutter. The anisotropy is achieved by oblique sputtering, and low skew is achieved with no magnetic field applied parallel to the substrate. The deposition technique disclosed in Japan Patent No. 4352104 is executed by a sputtering apparatus that includes an elongated target held in a manner rotatable around a first rotational axis, a substrate held in a manner rotatable around a second rotational axis disposed in parallel with the first rotational axis, and a drum shutter which is disposed between the target and the substrate and which is rotatable around the first or second rotational axis. During deposition, the substrate and drum shutter are rotated. Sputtered particles pass through the opening of the drum shutter and are deposited on a portion of the substrate on the stage.
After the deposition of up to half of the thickness of the film to be formed is complete, with the substrate being rotated by 180 degrees around a third rotational axis perpendicular to the second rotational axis, deposition is further performed while the substrate is rotated in the opposite direction around the second rotational axis. This deposition technique can further improve the film thickness uniformity in comparison with deposition techniques in a single direction. The improved film thickness uniformity by the rotation direction is achieved by the accurate control of substrate and shutter angular velocities during deposition.
However, in JP 2005-526179 A, higher induced anisotropy by oblique sputtering is not taken advantage of. Control of sputtering incidence angle and distribution is not addressed.
Unfortunately, in the apparatus of Japan Patent No. 4352104, because the distance between the source of particles (target surface erosion area) and the deposition area on the substrate may vary during deposition, the Hk in-wafer non-uniformity can still be large. The separation between the wafer area being deposited onto and the shutter opening is also not constant. That is, the deposition geometry varies for every portion of the wafer. It is also difficult to control transverse uniformity as the shutter opening shape is fixed. The optimum shape of the opening is expected to be dissimilar for different target materials as well as process conditions such as gas pressure and cathode power.
Therefore, it is an object of the present invention to provide a sputtering apparatus for obtaining good Hk uniformity, low skew values, and good thickness uniformity to optimize deposition geometry with respect to the target and the substrate during the deposition, even when forming at various angles in order to obtain good anisotropy strength, or even when using various kinds of target materials.